1. Field of the Invention
The present invention relates to an optical system for optical recording, reproducing or erasing in an information-recording medium, and more particularly to a method and apparatus for tilt detection in said optical system.
2. Description of the Background Art
FIG. 1 is a view showing a conventional optical pick-up apparatus for an optical system.
As shown in FIG. 1, the optical pick-up 200 includes a laser diode 260 for generating light irradiated on the information-recording medium, a collimate lens 250 for changing the light generated from the laser diode 260 to a parallel light; a beam splitter 220 for refracting the optical axis of the parallel light to 90°, an objective lens 210 for collecting the refracted light and irradiating the refracted light on a track of the information-recording medium; a light-receiving lens 230 for forming an image of the reflected light on a photodiode when the light irradiated on the track of the information-recording medium is reflected from the information-recording medium; and a photodiode 240 for outputting a light amount signal corresponding to the light amount of the reflected light.
FIG. 2 is a view showing a diffraction pattern of the light beam when there is not a tilt or a radial tilt.
FIG. 3A and FIG. 2 are a view showing a diffraction pattern of the light beam when there is a tilt or a radial shift, respectively. As shown in FIG. 3A and FIG. 3B, the diffraction pattern of the light beam shows an asymmetric distribution. In this respect, the darker portion of the drawing indicates a lesser amount of light.
FIG. 4 is a view showing four divided photo diodes (A, B, C and D) in accordance with the conventional art.
As shown in FIG. 4, when the push-pull method, i.e. one of methods for detecting the degree that the currently irradiated light is deviated from the track, is employed, a push-pull value is expressed by a difference between the sum of the light amount detected from the photo diode 240 (A, D) and the sum of the light amount detected from the photo diode 240 (B, C). When it is taken as a formula, it can be expressed by p_p=(A+D)−(B+C).
When the light is irradiated at an accurate position of the track, as shown in FIG. 2, since the left and right light amount of the diffraction pattern are the same, the push-pull value is ‘0’. On the other hand, when irradiated off the accurate position of the track, the diffraction pattern shows an asymmetric distribution so as to yield a positive or a negative value of the push-pull value. Accordingly, it can be determined how far the presently irradiated light is deviated from the track according to the sign.
However, even though the light is accurately irradiated on the track of the information-recording medium 100, if the information-recording medium 100 is slanted, the light amounts of the left and right sides are different to each other. And when there happens a radial shift with an objective lens moving along the track, the diffraction pattern shows an asymmetric distribution so as to yield a positive or a negative value of the push-pull value. Accordingly, it is not possible to know from the resultant push-pull value whether the asymmetric distribution is owing to the disk tilt or the radial tilt.
In other words, as shown in FIGS. 5A and 5B, since the tilt amount for the push-pull value and the slope of the radial shift amount are similar, there is little difference between the push-pull value according to the tilt amount and the push-pull value according to the radial shift amount.